THE STRUCTURE AND DEVELOPMENT OF THE VISUAL AREA IN THE , PHACOPS RANA, GREEN.

BY JOHN M. CLARKE.

To students of the fossil Arthropoda it should be a matter of congratulation that so great success has been achieved by palaeontologists in solving the problem of anatomy and develop- ment in the extinct crustacean order, the Trilobita. Though much may remain to be done, too great esteem cannot be accorded to those who have contributed to what has been ac- complished ; Eichwald, Burmeister, Volborth, Quenstedt, Rich- ter,’ Barrande, Hall, Billings, Ford, Walcott, Woodward, Mickle- borough, Matthew, Pack rd. To the labors of these men is due our knowledge of the t evelopment of the trilobite from the ovum to maturity; of its delicate locomotive and respiratory apparatus ; and somewhat of its reproductive, alimentary, and muscular anatomy. The present paper endeavors to throw some light upon the structure and development of the eye in a typical representative of an extensive group of , Pkacops rana, Green. In the study of this organ abundant material consisting of several thousand specimens has been accessible, in the majority of in- stances only those being utilizable in which the lenses of the eye

1 It may be well to call attention to the fact that Richter’s single observation upon the ventral anatomy of Phucops, which has been overlooked by later investigators, is of much greater significance than the author himself accorded it. In the Beitragzur Palaonfologir des Tlilrangt-r Waldes, 1848, PI. 2, Fig. 32, is given an enlarged new of a transverse section through one-half the thorax of a De- vonian Phucops (species not given). As this work may not be generally accessible, the figure is here reproduced; and =?= although the author states (op. cit. p. 20) that the section serves to establish Bunneister’s conception of the ventral anatomy of the trilobite, in the light of Walcott’s demonstration of the spiral branchk in Calymcne scnaria it appears that what is represented here is a secticn of one of these appendages. I may add that I have also detected evidence of these spiral branchiae in Plucops rana. 254 [VOL. 11. have been so perfectly retained as to allow of enumeration. These specimens have been derived from the shales and lime- stones of the Hamilton group at various localities in Western New York, those best adapted for the purpose of sectioning being from the basal limestones near Centerfield, Ontario County.

THECHARACTER OF THE VISUAL AREA in the trilobites is twofold; (a) it may be covered by a smooth, continuous epi- thelial film or cornea, through which the lenses of the ommatidia are visible by translucence, and (b) the cornea may be transected by the protrusion of the scleral and limited to the surfaces of the ommatidia. To the first group belong species of the genera Asaphus, IZZmws, CaCymeneF Homa Zonotus, Proetus, Cyphaspis, Acidaspis, Lichas, and others ; to the second, the single exten- sive family, Phacopidcz, with its genera, Phacops and DaZma- nites (? Haves; vide conclusion). The first group may be des- ignated by the term HoZochrouZ; the second group by the term Schizochvoal.

PHACOPSRANA is one of the most abundant and characteristic species of the Hamilton faunas. Though widely distributed in the formations of this age throughout the United States and Canada, it is not known with certainty to have been present in faunas older than those of the Hamilton, and it does not appear to have continued its existence after the displacement of the Hamilton faunas. A detailed and very complete description of the species, accompanied by copious illustration, is given in the

1 I have found the term ommatidium, proposed by Carritre for the little eyes or ocular elements in the compound eyes of and Mollusks, a very convenient and significant term, but may use it with a little license, as I do not regard the eyes included in the second of the above groups as properly compound. The term rcleru as here used may be open to some objection. It is applied to the interstitial test between the ommatidia, and is preferable to the expression cornk ojaquc of Bar- rande. Professor Edward Orton, of Columbus, Ohio, has allowed me to examine a very young individual of Culymcnc senaria in which the lenses are relatively very large, and are strongly suggestive of the character of the lenses in Phacops, although in the adult of this species they are so small as scarcely to be detected. This specimen suggests the query whether in the holochroal eyes the lenses may, with the advanc- ing growth of the , become apparently smaller, from close juxtaposition or other cause, and also indicates the possibility that the difference in the holo- and schizochroal eyes is not as great as it now appears. No. 2.3 EYES OF ARTHROPODS. 25 5 monograph of North American crustacea, constituting Volume VII. of the Palzeontology of New York. In this place are also to be found satisfactory figures of the eye in various modes of preservation.

COMPOSITIONOF THE VISUALNODE. The eye is composed of the visual surface, which is normally a hate segment of the surface of a cone, but often in senile individuals is inclined to sphericity ; this surface is buttressed on the glabellar side by a strong palpebral lobe, which is pro- duced to and slightly beyond the upper edge of the visual sur- face, forming a distinct palpebranz. The lower edge of the visual surface is bordered by a ridge, which becomes broader and more conspicuous outwardly, and may be called the orbital ridge. The Zenses as seen from the upper surface are convex, some- times being translucent, especially when they have been filled with crystalline calcite, or have been slightly separated from the matrix; they are circular in outline, although the cavities in which they lie (ZeensaZpits) appear in old individuals to be hex- agonal. This appearance is due to the undiminished growth beyond maturity of the sclera, which crowds upon and' overlaps the edges of the lenses on all sides, deepening the lensal pits. The general arrangement of the lenses is in alternating vertical rows or quincunx. For convenience, however, in the accom- panying statements of enumeration, the lenses are regarded as arranged in diagonal rows parallel to the lower posterior margin of the visual surface, and are numbered from this line consecu- tively. It appears probable that this is also the order followed by nature in increments to the number of lenses, from the time of the formation of the primary row of ommatidia (vide seq.) onward to maturity. Under this arrangement the last row in enumeration is that ending in the upper anterior angle of the visual surface. The namber of these mws is variable, in the majority of cases being nine, in comparatively few individuals of average size and richly supplied with lenses, being ten, in extremely rare instances eZeven, only a single example of average size showing so many ; in very young individuals the number of rows is but eight. The youngest specimens observed show no less than this number, and it seems probable from other considerations that in still CLARKE. [VOL. 11. 2 56 earlier stages of growth the number of diagonal rows was not much less than eight. The number of lense; constituting the visual surface of each eye is variabZe, bzct not irregzclaarb so. The smallest number noted is thirty-three, in a very young individual having but eight rows ; the greatest is eighty-eight, occurring in the single example mentioned, which bears eleven rows. Again, the number of lenses in sziccessive rows is vanabZe, but only so within certain well-dejned limits. In order to make this point clear, the following ten enumerations are presented, taken at random from a list of above three hundred tabulated eyes.

eG Rows. E ------w2 1 2 3 4 5 6 7 8 9 10 ------I 1 8 9 I0 I0 9 8 6 4 2 66 2 I I0 11 I1 I0 9 7 6 4 2 71 3 4 9 I1 I0 I0 9 8 G 4 2 74 4 9 I0 I1 I1 9 8 6 4 2 70 5 I 9 I0 I1 10 9 8 6 4 2 70 6 8 I0 I0 9 9 8 6 4 2 66 7 8 9 9 I0 9 8 6 4 2 65 8 I0 I1 I1 I0 9 8 6 4 2 71 9 3 9 I0 I0 I0 9 8 6 4 2 71 10 6 I0 12 I2 I1 I0 8 6 2 81 ------4 - - It appears from this table that whatever may be the total number of rows in any eye, in the last four rows of lenses, and generally in the fourth from the last, the number of lenses is subject to very little variation, while the number in the first two or three rows may vary greatly. This variation is partially explained by the following fact : It will be found upon examina- tion of an average eye that the anterior edge is normally nearly vertical ; the vertical row following this edge is composed of four lenses. It is impossible for any lenses to be added to the anterior extremities of the diagonal TOWS terminating at this anterior margin, for new lenses are added only from the lower and upper margins of the visual surface (see further on). In No. 2.1 EYES OF ARTHROPODS. 257 immature eyes a greater variation is noticeable in the last rows, as seen in the following examples of surfaces bearing but eight rows of lenses.

(1) 4. 5. 5. 5. 5. 5- 4. 2. 35 (zj 4. 4. 4. 5. 5. 5. 4. 2. 33 (3) 6. 6. 6. 6. 6. 5. 4. 2. 41 (4) 7. 9. 7. 4. 6- 5- 3. 1. 42

In these instances the eye has not attained its full growth in height, which would preclude variations in the last rows. Conversely, as the first four or five rows of lenses terminate on the lower margin of the visual surface, and as additions to the number of lenses are made most abundantly from this area, the number in these rows is constantly varying. It may be here stated, that with the exception of the right and left eyes of the same individual, no two eyes in all the specimens enumerated have shown the same number of lenses in all corresponding rows. A definite relation exists 6etween the number of Zenses of the eyes mid the size (i.e. age) of the animaZ. This fact has been established by recording with each enumeration of lenses a single measuxment which would serve as an index of the stage of development attained by the animal. The measurement taken is the basal width of the cephalon. Phacops ram is rarely found with all the parts in articulation, and still retaining the lenses with sufficient distinctness for enumeration. Detached cephala are abundant, and it serves every purpose to take the indicia1 dimension from this part of the animal ; it is, moreover, found that the peculiar form of the cheek renders this dimen- sion of the head less liable to variation from flattening in the shales than the longitudinal measurement. Comparison of all the specimens enumerated gives the following results :- The average number of lenses in individuals having a cephalic width less than 10 mm. is 44 Between 5 and 15 mm. is 56.5 " 10and 20 mm. is 69.5 'I 15 and 25 mm. is 73 " 20 and 30 mm. is 71 " zj and 35 mm. is 66 258 CLARKE. [VOL. 11.

Between 30 and 40 mm. is 62.5 " 35 and 45 mm. is 62.1 From 40 mm. upwards, 58 The cakdated average basal cephalic width in this species, deduced from measurement of 1518 cephala, is 22.8 mm. The material from which this average is derived was unselected, much of it collected without reference to quality or size, and is fairly representative. I therefore venture the statement that the average Pkacops rana has a width across the posterior margin of the cephalon of approximately 22.S mm. It is, moreover, probable that 22.8 mm. is approximately the dimensional index for the averag-e nomzal ad& of this species. In all specimens of the entire animal which have passed under observation, varying in axial length from 10mm. to IOO mm., no evidence has appeared of any developmental change in the suc- cessive stages of growth, except in the increase and diminution of the number of corneal lenses. Save in this one respect the species assumed all the features of maturity at a very early point in its history; and the data given above conclusively indicate that in this feature, also, maturity was attained with this stage of growth. The important conclusion here drawn is that the number of Zenses increases from youth to matzcrity (di- mensional index approximately 22.8 mm.), and decreases from maturity to seniZity. Two questions immediately arise from this inference ; (a)how is the number of lenses increased? and (b) how is it diminished? These points will be adverted to in a following section.

STRUCTUREOF THE LENS. Sections across the visual surface show that the lenses are unequally bi-convex, the curvature being greatest on the proxi- mal surface. This inferior surface is perforated by a central circular aperture. Vertical sections of the lens when favorably preserved, also show this envelope as a simple, thin, distinctly black or brown corneous film ; and in natural casts of the inter- nal surface of the visual area, the ommatidial cavities are repre- sented by a series of shallow cups standing on short pillars, and each bearing at its centre a little ball, which is the filling of the interior of the lens. These lenses are consequently cornea2 and No. 2.1 EYES OF ARTHROPODS. 259 hoZow. It appears, also, in sections that itz the matwe kns the cornea is discrete from the sclera, lying in juxtaposition with and held in place by it, but in nowise continuous with it. This fact is also frequentiy apparent in specimens from which the sclera has been removed by solution, leaving the corneal lenses standing on little pillars of the matrix, which has filled the ommatidial cavities. Other specimens just as frequently show the converse, the lenses being removed, while the sclera is retained. There is evidence which I deem worthy of considera- tion, that these corneal lenses, during the life of the animal, were not empty, but filled possibly with some viscid or spissate humor. This evidence is of the following character :- The cornea itself is thin, and of even calibre throughout its extent. The casts of the corneal cavities, such as are shown in Figs. 25 and 26, as little balls lying in cups, are not of sufficient size to have occupied all the space within the cornea. A speci- men of Phncops of a species closely allied to Ymza (Ph. cyistata var. p$a Hall), from the decomposed phtanite of the Cornifer- ous limestone, seems to demonstrate this fact. This fossil was evidently originally preserved in calcic carbonate, which not only replaced the entire crust, but filled the ommatidial cavities, and the posterior cavity.of the corneze as well. This calcic carboiate was subsequently removed from within, and its place partially taken by silica, and when exposed to more rapid decomposing agencies, the remainder of the calcic car- bonate was removed, leaving the fossil so preserved that the cornea and a thin film over the entire external surface of the sclera has been taken away, the remainder of the test being replaced by silica. The external surface of each lensal cast remains convex, but on carefully removing a little of the decom- posed rock from beneath the position of the cornea a vacant space appears, which corresponds to the corneal cavity as repre- sented by the ball-in-cup casts. To elucidate this point, see Plate XXI., Fig. 5, and explanation thereof, Again, certain well-preserved sections from the limestone show a distinct difference in the character of the matrix fill- ing the outer and inner cavities of the cornea, that in the outer being of lighter color, and more translucent (? subcrystalline), while that in the inner is the opaque mud of the sediment. More evidence upon this point is very desirable, but enough 260 CLARKE. [VOL. 11. has been seen to indicate the fact that the cavity of the cornea was not simple, but compound. (May the posterior cavity-fillings represent the position of the anterior extremities of crystalline cones ?)

MULTIPLICATIONAND DIMINUTIONIN THE NUMBEROF LENSES. The lenses of the visual surface are not all of the same size in any of the stages of growth observed. The size of the fully developed lens varies accGrding to the individual development of the animal ; i.e., the larger the animal, the larger the lenses ; but in any given subject some lenses are to be found which are below the average of size for that eye. These small lenses are found at the extremities of the diagonal rows which terminate on the posterior portions of the upper and lower margins of the visual surface. The inferior size of these lenses is due in part to unlike causes. Of these the principal cause is (a)that they are new and imma- ture lenses added in regular order to the ends of the rows of older lenses. It has not been as yet satisfactorily determined whether the increment of new lenses may take place at either upper or lower extremity of the diagonal rows, although the small lenses occur indifferently at either end. There is no reason to doubt that this addition does take place at the lower extremities, but on account of the close juxtaposition of the palpebrum to the upper margin of the visual surface, it may be questioned if at any period of growth sufficient room is allowed in this region for additional lenses. A secondary reason for the small size of the lenses is (b) the constantly increasing size of the interlensar sclera after matu- rity, which gradually envelops the lenses especially along the upper margin of the visual surface, where, by coming into con- tact with the increasingly prominent palpebrum, the lenses are often nearly concealed. To what degree the small size of the lenses along this upper margin may be due to the overgrowth of the sclera, and how much to the possibility of their being newly developed, it has been impossible to ascertain, but that it is due to a certain degree to both causes, is shown by the following facts : (I) immature No. 2.) EYES OF ARTHROPODS. 26 I eyes, in which the sclera has attained no excessive growth, very often show these small lenses along the upper margin, and they would, therefore, appear to be developed there; (2) it has already been shown that after the average normal mature growth of the animal has been reached, the number of the lenses becomes less with advancing senility. This fact must be explained either by the gradual envelopment of the lenses of the upper margin by the sclera and palpebrum, and their entire concealment within the substance of the latter, unless it is pos- sible that atrophy of the ommatidial nerve branches and con- comitant reabsorption of the lenses takes place with advancing old age. From the examination of eyes limited to eight rows of lenses, it appears that with this number of rows there may be consid- erable variation in the number of lenses, as seen on the plate (Fig. 9, thirty-one lenses ; Fig. 10,forty lenses). These figures also indicate the fact, that with a constant diminution in the number of lenses from the upper and lower extremities of the rows, the eight diagonal rows would ultimately be reduced to a single or double longitudinal row parallel to the margins of the visual surface. Hence, without overmuch hypothesis, the pri- mary lenses probably appeared in a single or double row, a visual line parallel to the margins of the orbital node.

DEVELOPMENTOF THE LENS. There is sufficient evidence at hand for the statement that that portion of the ommatidial cavity which penetrates the test arises from an evagination from the internal surface of the test accompanied by a corresponding but very shallow invagi- nation from the upper surface. Natural casts of the internal sur- face of the visual area not infrequently show minute lensar cav- ities at the ends of the rows of lenses, which appear not to have penetrated to the upper surface, and bear at their summits no impression of a corneal surface or corneal cavity, as do the other lenses in the same eye. It would be inferentially true that the cornea is developed from the attenuated integument (cuticular epithelium), and is the specialized film of the test left between the depressions from its lower and upper surfaces, eventually becoming discrete. 262 CLARKE. [VOL. 11.

STRUCTUREOF THE SCLERA. The interlensar sclera is continuous with the test, and its structure is in all points identical with that of the test. The vertical tubules and smaller tubulipores, with which nearly every part of the test of Phacops rum is densely perfo- rated, are plainly visible in every section of the sclera, no dif- ference in the structure of the parts being discernible, although the thickness of the sclera is somewhat less than that of the adjoining portions of the test; however, the thickness of the test is of necessity very variable in different parts of the ani- mal. Not infrequently eyes have been observed, preserved as casts in decomposed chert, in which the tubules of the sclera are represented by delicate rods traversing the vacant space left by the removal of the integument.

ABNORMALITIESin the arrangement of the corneal lenses are of comparatively rare occurrence. They appear to be due, in every instance observed, to the failure of a lens to develop at the proper time and place in its own row, but in no case has a lens appeared so out of place as to be intercalated between rows. Marked abnormalities, such as that represented in Figs. 11 and 12, are usually confined to one of the two eyes. It is, however, not uncommon to find the right and left eye differing in the number of lenses in the corresponding rows, either with or without affecting the total number of lenses. In the follow- ing example the total is the same, although the arrangement differs :- Right eye. 6. 8. 9. 9. 9. 7. 6. 4. 2 = 60 Left eye. 6. 8. 10. 9. 8. 7. 6. 4. 2 = 60 In another example both the number in the corresponding rows and the sum total differ:-

Right eye. 5. 7. 8. 7. 7. 6. 4. I. 2 = 47 Left eye. 5. 7. 9. 8. 7. 7. 6. 4. 2 = 55 These instances of irregular development may be due to pathologic or other organic conditions of the animal ; perhaps, aiso, in part to external influences. No. 2.1 EYES OF ARTHROPODS. 263 NOTE.-No satisfactory evidence of crystalline cones within the ommatidial cav- ities has been ascertained, and it is not surprising that these bodies, which undoubtedly existed, were removed with the soft parts of the visual organ. With respect to this feature the sections of the eye of ?'hacops given by Barrande (Sysfime SiZuvien du Cenfre de la BohBme, Vol. I., P1. 3, Figs. 15 and 16), and reproduced by Zittel (Haiadbuch dw Patontolog'e, 1885), are misleading. The fillings of the ommati- dial cavities are so shaded as " to indicate prisms " (compound) " corresponding with each lens," and extending very far inward without diminution in width. Such structure finds no correspondence in the eye of the living , and is prob- ably to a large degree schematic and imaginary. The structure of the lens, as we have found it, is also essentially different from that represented by Barrande.

MODES OF PRESERVATIONOF THE VISUALSURFACE.

a) The cornea and scZera are normaZ& presemed. (Fig. I.) This is the usual mode of preservation in the limestones where the original substance of the test has been preserved in calcic carbonate, though leaving so considerable a portion of the organic matter as to give a black and lustrous surface. Such specimens retain the minute structure of the test most perfectly and are most satisfactory for sectioning. 6) The corrrea is removed and the sclera retained (Fig. 2.) This is a rare mode of occurrence noticed only in specimens from the shales. c) The scZera is removed and the cornea retained. (Fig. 3.) In these examples, which occur in the shales and weathered limestones, the corneal lenses stand supported on the summits of pillars of matrix. It is not an uncommon mode of occurrence. d) Both cornea and sclera are removed (Fig. 4), leaving pillars of the matrix with cup-shaped upper surfaces, each bearing a little ball at the centre. This condition is often observed in the decomposed limestones and phtanites. e) An external film is removed from the entire visual area, destuoyiq- the coynga (Fig. 5). A single example has been observed in which the entire test was apparently originally pre- served in calcic carbonate. Subsequently this was removed from within and gradually replaced by silica, with the exception of the thin outer film, which afterward was entirely removed, leaving the space it occupied vacant. f) Silica deposited as a thin jiZm upon, or rejlacizg a thin fiZm of the external and internal surfaces of the test, and aZZ the rest of the substance of the test and the matrix removed (Figs. 6 and 7). In this condition the visual surface is a mere shell appearing as 264 CLARKE. [VOL. 11. when normally preserved, but the corneal lenses are hollow, and the sclera represented by a thin wall of silica. This condition is sometimes modified by the removal of the entire upper sur- face of the visual area, generally by its adherence to the outer part of the matrix, leaving only vertical tubes representing the ommatidial cavities.

While the foregoing observations and the essential conclu- sions therefrom in regard to the structure of this phase of the trilobite eye, agree in some respects with the opinions of earlier writers upon this topic, there are many important points of dif- ference and various features of structure which have not before been noticed. I therefore give a brief historical review of the observations upon the subject. Quenstedt, I 837 (Wiegmann’s Archiv fiir Naturgeschuhte, Vol. I., p. 340), was the first to recognize two distinct types of structure in the eyes of trilobite, and divided them into I. * Aggregated eyes having a facetted cornea. 2. Aggregated eyes having a smooth cornea. In both groups the cornea was regarded as the direct continu- ation of the superficial layer of the test of the cheek. The first type of structure was represented by the eye of Phacops Zaatifrons, Bronn (ie., the Phacopidae). The second type was exemplified by IZZcznus crassicau&, Dalman (i.e., holochroal eyes), in which the facets are said to be in relief upon the internal surface of the general corneal (visual) area, each facet being formed by a lens or crystalline body, behind which lies a vitreous body, penetrating deeply into the organ. In these the cornea was regarded as composed of two distinct layers, of which the outer is quite smooth, the inner very finely reticulate. Burmeister, I 843 (Organization der Tdobiten, p. IS), regarded the structure of the holochroal eye as directly comparable in all respects to that of the eye in Branchipus stagnaZis, and indorsed Quenstedt’s view of the compound corneal layer, while admit- ting but a single type of structure. He assumes with respect to Phacops that the cornea must have been more destructible than in the other trilobites, and by its removal the facetted sur- face exposed.

* As I have not had access to this work, I am compelled to take the summary of these observations as given by Barrande (Syst. sic., p. 133). No. 2.3 EYES OF ARTHROPODS. 265 Barrande, 1852 (Systtme siZurien du centre Bohtme, Vol. I., p. 185), recognizes three distinct types of structure, two of which are similar in breadth to those of Quenstedt, though differently interpreted. The third type of structure is exemplified in the genus Harpes, the eye of which is considered as an aggregation of oceZZi (two or three in each eye). For the schizochroal eyes Barrande establishes the fact that the sclera (cornee opaque) of the visual surface is identical with that of the test of the head, and continuous with it. He also regards the existence of a transparent cornea covering the entire surface as suggested by Burmeister, as probable, but admits that he has been unable to assure himself upon this point. For the holochroal eyes the en- velope of the visual surface is shown to be of different character than that of the test, and consists of a general cornea covering the entire visual area. His observations do not seem to have led to a decisive conclusion in regard to the simple or compound character of this cornea. Packard, 1880 (ThStnutare of the Eye of Trilobites: Ameri- can NatuvaZist, p. 503), quotes a resume of the status of the dis- cussion as given by Gerstacker in Rronn’s CZassen and Ordnungen des Thierreichs, which is virtually a reiteration of Barrande’s opinions. The “ocelli” of Harpes are regarded as aggregate eyes, not comparable with the simple eyes or ocelli of LimuZus and the Merostomata. The eyes of Barrande’s first and second groups are considered as true compound eyes and not aggre- gated eyes; no essential difference is recogni; ed in the form and arrangement of the corneal lenses of Phnco is and Asaphus, and the distinctions pointed out by Quenstedt a id Barrande are considered artificial. The sections used by Professor Packard in he comparative study of the trilobite eye appear to have beea altogether of holochroal forms. A close correspondence in structure is de- monstrated between the eye of Asaphus and that of LimuZus. In the LimuZus eye the lenses are covered by a continuous cor- neal layer, which does not make.itself apparent in Professor Packard’s sections of Asapiius, although it undoubtedly exists. It may be questioned whether the conclusion drawn from this comparative study is not too broad, viz., that the trilobite eye is organized on the same plan as that of LimuZus.1

1 Professor Packard’s observations upon the eye of Asaphus show no indication of the existence of interstitial epithelium between the lenses, an extremely important 266 CLARKE. [VOL. 11.

CONCLUSION. The study of the eye in Phacops rana as here presented al- lows the statement of the following points :- I) The schizochroal eyes of the Trilobites are aggregated and not properly compound eyes. The visual organs of Harpes may prove to be of similar character. 2) The scleral portion of the visual surface is of the same structure as the test, and is a direct continuation of it. 3) No evidence appears of any continuous corneal layer cov- ering the entire surface. 4) The corneal lenses are wholly discrete from the epidermis, but are of epidermal origin. In the addition of new lenses to the visual surface, they appear to arise from a thinning of both surfaces of the integument. 5) The corneal lenses were hollow or filled with some matter not homogeneous with the cornea itself. 6) The corneal lenses, and therefore the ommatidia, are added to the visual surface with advancing age until the mature growth of the individual is attained ; thereafter they diminish in number with increasing senility. 7) The addition of corneal lenses occurs regularly at the ex- tremities of the diagonal rows. 8) No evidence is preserved of crystalline cones in the om- matidial cavities, though they may have been removed in the decomposition of the soft parts of the eye.

In conclusion I wish to call attention to the primitive struc- ture of the eye exhibited in a Devonian subtype which is provis- ionally referred to the order of the Phyllocarida. Dr. J. S. Kingsley in the final paragraph of a valuable paper on the (‘Development of the compound eye in Crangon ” (Journal of MoqhoZogy, Vol. I., p. 63), has written : “The observations as yet recorded are not sufficient to throw any great light on the phyllogeny of the Arthropod eye ; still one or two points may be feature of difference from the schizochroal eyes. My own study of the holochroal eyes has not been as careful as I hope to have the opportunity of making it, but it may be observed that sections of the eye in PYO~~USRavi seem to indicate a very tenuous interlensar sclera; moreover, the immature Cdymene senaria referred to in a previous footnote shows evidence of such interlensar integument. No. 2.1 EYES OF ARTHROPODS. 267 spoken of. The mere fact of invagination must be regarded as indicating an ancestral condition, but what this condition was is uncertain. The pit or groove must have had sensory func- tions and either wall ” (retinogen and gangliogen) “must for a time have been like its fellow, as is shown by its having similar nuclei, and by the similar development of rows of nuclei.” In the species Mesothyva Oceani, Hall, a member of one of the faunas of the Portage group, and one of the largest known repre- sentatives of the Phyllocarida, the eye consists of a simple deep pit at the summit of the optic node. There is no evidence that this pit contained a series of lenses, but it is highly probable that it is an otherwise embryonic character retained at maturity, and may serve as the ancestral condition of the Decapod eye sug- gested by Dr. Kingsley. That there is Decapod blood in the Phyllocarida has been disputed by Packard, the author of the group term, but Decapod affinities are strongly indicated by the recently described Devonian genus of Phyllocarida, Rhinocaris, (PaZontoZogy of New Yo&, Vol. VII., 1888).

1Thii refers to the primary optic invagination in the embryo, for the formation of the entire visual surface, first pointed out by Locy (BuZl. Mus. Comp. ZoX 1886). and verified by Kingsley (op. cit.). The term is not used in a sense similar to that in which it has been employed in this paper in referring to the formation of the corneal lenses. 268 CLARKE. [VOL. 11.

EXPLANATION OF PLATES. FIGS.1-7. Schematic representations showing the different modes of preservation observed in the visual surface of Phacops. Each figure represents a single lens with the lensal or ommatidial cavity, and the adjoining sclera or its equivalent space. I. The cornea and sclera normal& retained, the lensal and corneal cavities being Wed with matrix. The lens is represented with its posterior cavity having the rela- tive size indicated by the specimens figured elsewhere on the plate, and the anterior corneal cavity is characterized by radiating lines which are intended to show the difference often apparent in the character of the matrix filling the cornea. 2. The cornea removed and fie sclera retained, the matrix showing the position and size of the posterior corneal cavity, but retaining no indication of the anterior cavity filling, which is removed with the cornea. 3. The cornea refained and the sclera removed, the lens standing at the summit of a pillar of matrix which represents the ommatidial cavity. 4. Bofh cornea and sclern removed, leaving pillars of matrix with a cupshaped summit, in the bottom of which lies a little ball. The pillar represents the ommati- dial cavity; the concave summit, the lower surface of the lens, and the little ball, the posterior corneal cavity. 5. An exfernal$ln~ is removedfrom both cornea andsclera, destroying the for- mer and leaving the filling of the anterior corneal cavity standing out prominently with almost the full size of the lens. In the single instance observed of an eye in this condition, the posterior corneal cavity is empty in all the ommatidia that were opened. The reason for this is not well understood. The sclera has been silicified, and subsequently decomposed, so that it is indistinguishable from the matrix. 6. The outer and inner walls of the visual area have been replaced by .firm of silica, and the rest of the calcareous matfer subseguenf& removed, leaving both cornea and sclera preserved as a mere shell. 7. The same condition of preservation, modified by the adherence of the cornea and outer wall of the sclera to the matrix outside the eye, leaving the walls of the ommatidial cavities adhering to the internal matrix as a series of short tubes. This mode of fossilization has not been observed in Pharops ratta, but is not uncommon in specimens of Phacops cvistafa, var. pipa, from the decomposed Upper Helderberg phtanite. FIGS.8-22. Schematic representations of the lenses of the visual surface. The curved surface is projected upon a plane, and the relative size and position of the lenses is retained. Whether the representation is from a right or left eye, the lower posterior margin is at the right of the figure, the diagonal rows being enumer- ated from this side, obliquely downward from right to left. All the figures are drawn to the same scale. 8. The visual surface of an extremely young individual measuring 6 mm. across the base of the cephalon; composed of 31 lenses in 8 rows, nearly all the terminal lenses being immature. The older lenses show a tendency to arrangement along a single or double transverse row, parallel to the margins of the visual surface. 9. An older eye, bearing 35 lenses in 8 rows, belonging to a young Phacops, hav- ing a cephalic width of 7 mm. At the upper extremities of the rows the lenses are all full-grown, and all immature at the lower extremities. 10. An eye composed of 40 lenses in 8 rows, and belonging to a young individ- ual with a cephalic width of g mm. No. 2.1 EYES OF ARTHROPODS.

XI. An eye with 42 lenses in 8 rows, belonging to a large individual, having a width across the cephalon of 34 mm. The lenses are nearly all mature, but are abnor- mal in their arrangement, showing a failure to develop properly at the upper extremi- ties of the rows after the third. 12. An eye of Phacops crisiah, var. pipe, composed of 50 lenses, and showing an abnormal arrangement, a single mature lens in the last row being situated high up in the base of the palpebrum. Had it developed in the vacant space in the last row but one, the arrangement of the lenses would have been normal for this variety. 13. An eye of the same variety bearing 52 lenses in 8 rows. Eight appears to be the normal number of rows in the mature eye of this form, the anterior vertical row, and the last diagonal row, consisting of three lenses. In the other diagonal rows there is apparently much greater variation than in the eye of Phucops rana. 14. An eye of Phucops ranu composed of 57 lenses in g rows, the head having a basal width of 52 mm. This is the eye of a senile individual, and all the lenses are of mature size with the exception of those on the palpebral margin. The number of lenses in the anterior vertical row is three instead of four, as in the normal adult. 15. Another senile eye, with 60 lenses in 9 rows, belonging to a very large speci- men, with a cephalic width of 70 mm. Here again the lenses are all of full size with the exception of those on the upper margin of the visual surface; the lenses of the anterior vertical row are also three in number. 16. An eye composed of 60 lenses in 9 rows belonging to an individual slightly above normal adult size, having a cephalic width of 28 mm. In the first row one interval has been skipped in the addition of the last lens. 17. An eye of normal size and development, bearing 70 lenses in 10 rows, the cephalon having a width of 24 mm. The first row consists of a single full-grown lens at a considerable distance from the posterior extremity. 18. An eye from an individual of the same size, having 70 lenses in 9 rows. 19. An eye composed of 71 lenses in 10 rows, the cephalon to which it belongs having a width of 27 mm. It essentially differs from the preceding only in the pres- ence of the single lens constituting the first row. 20. An eye with 75 lenses in 10rows, from an individual having a cephalon 28 mm. in width. 21. An eye composed of 88 lenses in 11 rows, from an individual measuring 17 mm. in cephalic width. This is the greatest number of lenses and rows of lenses noticed in this species. zz An average eye of Dalmanitrs Boothi, var. Calliteles, Green, consisting of 206 lenses in 29 rows. FIG.23. A left eye of Phacops runa, enlarged to 3 diameters, showing the arrange- ment of the lenses, the smaller size of several of the terminal lenses, and the tuber- cles on the integument of the palpebrum and orbital ridge. The lensar cavities are represented as much too sharply hexagonal; they should be more rounded and exca- vate. The figure is copied from the Pdzontoo[oby of New Yod,Val. VII., P1. 8, Fig. 6. FIG.24. A portion of the visual surface of a very old eye, taken from an individ- ual measuring 70 mm. across the base of the cephalon; showing the thick and deeply excavate sclera, the full-grown lenses dong the lower margin, and the small lenses at the upper margin of the area. Knlarged to 3 diameters. 1Tii;. 25. 12 nnturnl cast of the intrriinl surface of n portion of the visual area in Pharops crirhzt,z, vnr. f ;fa,ciihrgecl to 6 diameters. The sprcimeu sho\vs very hcnu- tifully the cup-sllapcd c:ists of tlic omnintidinl cavities, each with n littlz ball :It its cc'ii- tre represeiiting the posterior corned cavity. the upper exrreinitirs of the lsst PI,. XXI. CLARKE. [VOL. 11. 270 two vertical rows at the left, are two casts of immature lenses, in one of which the filling of the corneal cavity is just discernible; in the other the ommatidial cavity appears not to have penetrated to the upper surface. Several features of similar character are to be seen on parts of the specimen not represented in the figure. The portion of the eye represented is the posterior one-third of the right eye. FIG. 26. Two cavity-fillings from the same specimen enlarged to 15 diameters, showing the small size of the balls compared with the probable size of the entire cavity of the cornea. FIG.27. A section through the eye and adjoining parts of Phncops ram, showing the lenses and the interlensar sclera. Enlarged to 3 diameters. FIG.28. The same enlarged to 6 diameters, showing the continuity of the porifer- ous integument of the head with the interlensar sclera, the double convexity of the lenses, and the depth of the ommatidial cavities in the sclera. At the right of the first lens in the series is an internal depression on the integument which appears to indicate the position of a newly developing ommatidium. No evidence of a lens was visible at this point before the section was made. FIG.29. Three lenses with their interstitial integument, from the same specimen, enlarged to 10 diameters. There is a slight difference in the character of the matrix filling the cavity of the cornea, it being possible to distinguish the line between the anterior and posterior divisions of the space. The dark color of the lower portion of the sclera is due to an increase of pigment. FIG.30. A portion of the eye of Piiacops rana, from which the sclera has been removed by natural causes, leaving the cornere standing on pillars of matrix. En- larged to 12 diameters. FIG.31. A natural section of the eye of Phucops cristata, var.pipa, enlarged to 8 diameters. The sclera has been removed, and the doubly convex surface of the len- ses is well shown. FIG.32. The eye of Mesothyru Ocrani, Hall, enlarged to 3 diameters, showing the strong optic node with a simple, deep pit at its summit. The figure is taken from the PakzontoZo# of Nm York, Vol. VII., P1. 32, Fig. 2.